Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla)

Abstract

available online at hstp;(hww.idealibrary.com on I E )r-L c @ --_ Characters of cranial and postcranial osteology provide important data for examining the interordinal relationships of mammals. Understanding variation in the cranial and postcranial skeleton is necessary for adequately representing ancestral character states for each mammalian order. This paper provides a comprehensive description and discussion of cranial and post-rsnial osteology in one of the major perissodactyl lineages, the Tapiromorpha, the lineage including the extant rliinoceroses and tapirs and their fossil relatives. Tapiromorph skeletal morphology is described, and tapiromorphs axe compared to each other, to other perissodactyls, and to a number of other eutherians.Characters of potential phylogenetic significance are emphasized. A phylogenetic analysis of tapiromorphs using cranial and postcranial osteological characters provides a number of interesting results. The data do not support slacing Homogalar and Chalicotherioidea within Tapiromorpha. The data support several novel hypotheses regarding the phylogeny of rhinocerotoids, including a close relationship between rhinocerotids and amynodontids, relative t o hyracodontids. Finally, examples of the relevance of tapiromorph osteology to understanding perissodactyl origins are discussed, particularly new interpretations of certain primitive character states for perissodactyls.

The origin of chalicotheres (Perissodactyla, Mammalia)

“…Cladistic analyses have subsequently linked Lophiodontidae with chalicotheres in the Ancylopoda (Hooker 1984(Hooker , 1989Froehlich 1999), these together being sister group to the Ceratomorpha. In contrast, on cranial (non-dental) and postcranial characters, Holbrook (2001) found no evidence to relate Ancylopoda more closely to one rather than any other group of perissodactyls.…”

“…Heptodon and Karagalax were chosen as primitive ceratomorphs. Within Ceratomorpha, Heptodon is also regarded as a stem tapiroid (Dashzeveg and Hooker 1997;Holbrook 2001). Karagalax was originally described as an isectolophid by Maas et al (2001) but reinterpreted as a stem ceratomorph by Hooker and Dashzeveg (2003).…”

Earliest known unequivocal rhinocerotoid sheds new light on the origin of Giant Rhinos and phylogeny of early rhinocerotoids

“…The genus Hyrachyus is close to rhinocerotoids, but maintains a number of features primitively similar to basal tapirmorphs7. Although Hyrachyus was regarded as a rhinocerotoid genus in several recent studies789, this affinity can’t be warranted from inconsistent phylogeny9. Its taxonomy is strongly in need of revision, and whether it is a true rhinocerotoid still remains to be further explored in broader phylogenetic analyses.…”

The origin and evolution of mammals

“…As with perissodactyls as a whole, the paucity of modern species, no more than four in this case, is a small echo of a once much more diverse group (Schoch 1989;Holbrook 2001). The greatest diversity of tapiroids occurred during the Eocene, when there was an abundance of genera across Eurasia and North America.…”

“…The interrelationships between this order, other ungulate orders, and 'condylarths' continue to be argued over without agreement (e.g. Thewissen and Domning 1992;Fischer and Tassy 1993;Holbrook 2001).…”